1. Technical Field
The invention relates to a filter function diagnostic system for diagnosing the functions of a filter of internal combustion engine on which a selective reduction NOx catalyst is supported. This type of filter has a PM trapping function of trapping PM (Particulate Matter) in exhaust gas, and also has a NOx converting function of reducing NOx in exhaust gas, using ammonia as a reducing agent.
2. Background
A technology of providing a filter that traps PM in exhaust gas, in an exhaust passage of an internal combustion engine, is known. In some cases, a failure, such as erosion or breakage, occurs to the filter. If such a failure of the filter occurs, the amount of PM that flows out from the filter, without being trapped by the filter, is increased. If the failure occurs to the filter, or an abnormality, such as removal of the filter from the exhaust passage, arises in the filter, the PM released to the atmosphere is increased. Thus, a technology of providing a PM sensor in the exhaust passage downstream of the filter, and conducting an abnormality diagnosis on the filter based on the output value of the PM sensor has been developed. As one type of the PM sensor used for the abnormality diagnosis of the filter, a sensor that has a pair of electrodes as a sensor element, and outputs a signal corresponding to the amount of PM deposited between the electrodes, is known.
Also, an emission control system is known in which a filter, an aqueous urea addition device, a selective reduction NOx catalyst (which may be called “SCR catalyst”), and a PM sensor are arranged in this order from the upstream side along flow of exhaust gas, in the exhaust passage of the internal combustion engine. The SCR catalyst has a function of reducing NOx in exhaust gas, using ammonia produced by hydrolyzing urea injected from the aqueous urea addition device, as a reducing agent. In the emission control system constructed as described above, when the PM is detected by the PM sensor, injection of aqueous urea from the aqueous urea addition device is limited, as disclosed in International Publication No. 2013/042190
If the PM trapping function of the filter declines due to occurrence of a failure in the filter, the amount of PM flowing out from the filter is increased. Therefore, the amount of PM trapped between the electrodes of the PM sensor provided in the exhaust passage downstream of the filter is increased. As a result, the amount of PM deposited between the electrodes of the PM sensor becomes larger than that in the case where the filter is in a normal condition. Therefore, the PM trapping function of the filter can be diagnosed, based on the output value of the PM sensor obtained at a given time.